1. Field of the Invention
The present invention relates to heterojunction bipolar transistors in which an emitter region is made of InP having a wide band gap, a base region is made of GaInAsP having a narrow band gap, and an emitter/base region is made of a material with a band gap becoming gradually smaller from the emitter side to the collector side.
2. Description of the Related Art
Heterojunction bipolar transistors (HBTs) have been considered to have promising prospects in the field of microwave and high speed logical operations because of their excellent high frequency operations and switching performance.
To evaluate the operation speed of a bipolar transistor, a cut-off frequency f.sub.T is generally used. In an npn type transistor, the cut-off frequency is expressed in terms of a reciprocal of the transit time of electrons within the transistor device. Therefore, when it is desired to obtain a high cut-off frequency f.sub.T, it is necessary to shorten the electron transit time.
The electron transit time .tau. is expressed in terms of a sum of an emitter charging time .tau..sub.E, a base transit time .tau..sub.B, a collector transit time and a collector charging time .tau..sub.C.
The base transit time .tau..sub.B is constant regardless of a current density. The emitter charging time .tau..sub.E is decreased as the current density increases. When the transistor is operated at a high current density higher than the latter part of the order of 10.sup.4 A/cm.sup.2 or 10.sup.5 A/cm.sup.2, the emitter charging time .tau..sub.E takes a small value far below 1 psec.
Conventionally, major researches and studies on the HBTs have been directed to such a type of HBTs that use AlGaAs/GaAs material system as compound semiconductor materials. Recently, however, for the purpose of obtaining a higher speed and better performance, HBTs using the InP/GaInAsP material system have become actively studied.
In the InP/GaInAsP HBT, the emitter layer is made of InP having a wide band gap, the base layer is made of GaInAsP having a narrow band gap, the collector layer is made of GaInAsP or InP, the crystal mixture ratio of the respective layers being selected to be sequentially lattice matched with an InP substrate.
When the InP/GaInAsP material system is employed, it is possible to enlarge current amplification factor because InP has a small intrinsic surface recombination rate. Since the electron mobility is large within InGaAs and the band separation energy between .GAMMA. and L points in the band structure of InP is larger than that in GaAs, the velocity overshoot can be effectively utilized and thus the transistor can be operated at a higher speed. In addition, since the band gap of the InGaAs is smaller than that of the GaAs, the transistor can be operated with low power consumption. Therefore, it is generally considered that InGaAs is advantageous over GsAs in obtaining a high performance device.
Conventionally, for the purpose of simplifying epitaxial growth, the HBT using the InP/GaInAsP material system arranged as shown in FIG. 2, in which an emitter layer is made of InP, a base layer is made of InGaAsP, a collector layer is made of InP, and semiconductor composition abruptly varies at the interfaces between the respective layers. More in detail, the conventional heterojunction bipolar transistor comprises a semi-insulating InP substrate 1, an n.sup.+ type InP layer 2 as a collector contact layer, an n.sup.- type InP layer 3 as a collector layer, a p.sup.+ type Ga.sub.0.47 In.sub.0.53 As layer 16 as a base layer, an n type InP layer 7 and an n type Ga.sub.0.47(1-.beta.) In.sub.0.53+0.47 .beta. As.sub.1-.beta. P.sub..beta. layer 8 (1.0.gtoreq..beta..gtoreq.0) both forming an emitter layer, an n.sup.+ type Ga.sub.0.47 In.sub.0.53 As layer 9 as an emitter cap layer, these layers being sequentially laminated on the InP substrate 1 in this order. In addition, an emitter electrode 10, a base electrode 11 and a collector electrode 12 are formed in such a manner as to contact with these layers. In the drawing, reference numeral 13 denotes an insulating layer for isolation between elements, 14 denotes an insulating layer for isolation between electrodes, and 15 denotes a silicon oxide film.
With such an "abrupt" heterojunction where the semiconductor composition varies abruptly at the interface, when electrons flow from the emitter to the base, the heterojunction functions as a potential barrier, whereby the ON voltage of the transistor becomes high. Further, electrons flowing from the base to the collector are blocked by the potential barrier caused by heterojunction at the base/collector interface, thereby to greatly reduce the current amplification factor.
In order to remove the potential barrier in the conduction band of GaAs HBT or the like HBT, a so-called composition grading is employed in which the crystal mixture ratio of the semiconductor is gradually changed. With the composition grading, a smooth conduction band configuration is obtained in which electrons can be smoothly moved from the emitter to the collector.
For the purpose of reducing the base resistance, the impurity concentration of the base region is determined as high as possible. This, however, causes the mobility of electrons in the base region to become very small, whereby the electron transit time in the base region becomes large and thus the operation speed is reduced. In order to reduce the base transit time while the impurity concentration in the base region is kept at a high level, a method has been employed in which the semiconductor composition of the base layer is gradually varied to establish an electric field for acceleration of electrons moving within the base region.
However, although the above method of providing the electron accelerating electric field by varying the semiconductor composition of the base region is indispensable for realizing a higher-performance HBT, there has been proposed only such an HBT of InP/GaInAsP material system where the base layer has a uniform impurity distribution and a uniform semiconductor composition. Accordingly, as far as the above method is concerned, it is not known, for example, the spatial distribution of the semiconductor composition in this material system and the influence of the distribution on the performance of the element.
Further, in the HBT of the InP/GaInAs material system, how to provide a semiconductor composition grading at emitter/base and base/collector junctions, how to provide an electron accelerating electric field by varying the composition of the base region and how to vary the base composition are not known at present.
Under the above discussed situation, the prior art HBT of the InP/GaInAs material system could not have a satisfactory operation speed even though the emitter layer is made of InP which provides a large current amplification factor because of its small intrinsic surface recombination rate and the base layer is made of InGaAs which provides a high speed operation because of the large electron mobility within the InGaAs, it is impossible to obtain a sufficient operation speed.